Problem: Ishaan did 1 fewer sit-up than Christopher in the evening. Ishaan did 20 sit-ups. How many sit-ups did Christopher do?
Solution: Ishaan did 20 sit-ups, and Christopher did 1 more than that, so find the sum. The sum is $20 + 1$ sit-ups. He did $20 + 1 = 21$ sit-ups.